This invention relates to a system and method for determining aircraft weight-related data such as balance and weight.
Accurate aircraft load and balance information is crucial for safe and efficient aircraft operation. Federal regulation and good professional practice require that all private, military and commercial aircraft operators determine aircraft weight, weight distribution (balance), center of gravity, and aerodynamic center of lift before attempting flight.
Current commercial and military practice for calculating aircraft weight and balance is primarily an estimation process that is augmented by a few actual measurements. Aircraft Empty Operating Weight (EOW) is established by platform weighing at the time of aircraft manufacture or during periodic major maintenance. The EOW is then used as a base number for the daily operational practice of estimating gross aircraft operating and take-off weight. Cargo is generally weighed before being loaded on an aircraft. Cargo carriers operating from remote sites (tactical military transports, for example) have no platform scales available, and may be forced to estimate cargo weight. Other weight components (including aircraft balance), are estimated:
Passenger weight is typically estimated by assigning an average value (180 lb./male, 130-lb./female passenger) to each passenger.
Baggauge weight is estimated by assigning an average value to each piece, and then multiplying this average value by the total number of bags.
Fuel weight is estimated by converting the measurement of total fuel volume in the aircraft fuel tanks, based on an average fuel density value, and fuel totalizers are accurate only within xc2x12-3% of actual fuel volume.
Ground crew chiefs (or loadmasters) manually record the physical location on the aircraft where they loaded cargo and baggauge and then either submit this information to load agents for balance calculation or calculate aircraft weight distribution themselves by using manual (loading charts) or personal computer methods.
Military fighter aircraft operators generally sum the manufacturer""s advertised weight of the ordnance (missiles, bullets, bombs, electronic warfare pods, expendable fuel tanks, etc.) and add that number to EOW.
Tactical military fixed wing and rotary wing transport operators generally estimate their load weightsxe2x80x94especially when operating from unprepared fields or in combat conditions.
These weight estimation procedures are subject, to three types of error. The first is statistical error. For any estimation parameter, there is a built in statistical error margin: a certain percentage of weight estimates will be either too low (sacrificing safety) or too high (sacrificing aircraft carrying capacity). A second error source is passive human error: an individual gate agent, ground crew chief, loadmaster, dispatcher, load agent or ramp worker simply makes a mistake. A third type of error is cheating: an individual intentionally falsifies weight data. Instead of estimating aircraft weight and balance, there are two broad categories of systems which attempt to determine aircraft weight directly. Off-board systems make use of platform weighing in which the airplane is actually weighted. On-board systems typically measure changes in strut nitrogen or hydraulic pressures, or measure landing gear z-axis (vertical axis) shear and/or bending stress. U.S. Pat. Nos. 4,967,384 and 5,521,827 are directed to off-board weighing systems. On-board systems are described and claimed in U.S. Pat. Nos. 5,214,586; 5,540,108; 5,205,514; 5,257,756; 5,258,582; 4,507,742; 4,700,910; and 3,797,302. These patents typically use strain gauges or pressure transducers.
In order to accurately calculate aircraft weight using strain and/or pressure measurements, it is necessary to resolve all axial and cross-axial forces that act on an aircraft""s structure. Prior art systems such as those set forth above that have attempted to measure aircraft weight by measuring z-axis bending or shearing strain in the aircraft landing gear, or by measuring changes in hydraulic or pneumatic pressure in landing gear struts have failed to achieve the desired accuracy and reliability. The reasons are as follows:
First, the structural components of aircraft landing gear struts are not axially, radially and materially symmetric. The design asymmetries in material thickness, material types, and shapes result in an asymmetric distribution of force throughout the landing gear strut. This asymmetric distribution of force is, in effect, a series of cross-axis forces. Prior art systems that measure one-dimensional strain or pressure variations fail to consider this cross-axis phenomenon.
A second problem with systems that attempt to calculate aircraft weight by making single point z-axis measurements of force is that they fail to consider the fact that the sensor""s reference frame rotates or displaces (or both) as a result of changes in aircraft weight or as a result of aircraft movement. This bending of the aircraft landing gear under the strain of loading or movement creates, in effect, a series of cross-axis forces. The sensors that were-designed to sense z-axis bending in effect rotate, so as to be no longer oriented to the z-axis.
These asymmetries and cross-axis forces disappear when the weight measurement is made at the point at which the aircraft tires touch the ground. At that point, all the various components of force come together, so that when the aircraft is rolled onto a platform weighing scale, that scale measures the z-axis force exerted on the bottom of the aircraft""s tires. But it is impractical to position weighting scales at airports for daily operational use.
In one aspect, the method of the invention for calculating aircraft weight-related data includes affixing strain sensors to aircraft structural supports and/or landing gear struts to generate signals related to strut strain. Known loads are applied to the struts to determine a calibration matrix relating loads to the strain sensor signals. Weight-related data are calculated from the calibration matrix. Weight-related data may be aircraft weight or aircraft balance information. In one embodiment, the known loads are produced by providing tension between pairs of struts to provide off-axis loads.
In another aspect, the system according to the invention for determining aircraft weight-related data includes strain sensors affixed to aircraft support struts to generate signals related to strut strain. Apparatus applies known tension to the struts and computing apparatus calculates a calibration matrix relating loads to the strain sensor signals. The computing apparatus also calculates the weight-related data from the calibration matrix. Temperature sensors and accelerometers may also be affixed to the aircraft support structure for use in calculating the calibration matrix. A flight management computer may be provided for receiving the weight-related data. In this system, the off-axis loads resolve cross-coupling effects of bending stress on the support struts.
The on-board aircraft weight and balance calculation system of the invention thus produces highly accurate aircraft weight and center of gravity information and communicates this information to flight crews, the aircraft flight management computer, and to ground-based computers. Strain, temperature and acceleration sensors placed throughout the aircraft measure aircraft structural response to changes in aircraft weight. The unique calibration procedure of the invention resolves cross-axis coupling effects that influence measurement of z-axis strain. The procedure applies auxiliary axial and cross-axial loads, records structural response to these loads and then includes these measurements with structural bending strain, temperature and acceleration data in a mathematical procedure that determines aircraft weight.
The highly accurate weight and balance information provided to air crews and aircraft operators by the methodology of the invention reduces or eliminates costs associated with inaccurate or wrong estimates. These costs include decreased safety margins and lost economic opportunity. Safety is especially affected. The National Transportation Safety Board (NTSB) states that numerous airline and general aviation accidents are caused by overweight conditions. Economics is also a consideration. Current weight and balance estimation practices are labor intensive and costly. xe2x80x9cAwaiting loadxe2x80x9d delays cause missed connections, create passenger dissatisfaction, increase crew and fuel cost, sacrifice aircraft utilization, and waste perishable carrying capacity.
In contrast to the prior art, the present invention takes a systems-architecture approach to measuring bending strain and produces highly accurate and timely aircraft weight and center of gravity information. The invention positions groups of strain and temperature sensors throughout the aircraft structure in order to obtain independent information on aircraft structural response to horizontal, vertical and cross-axis stress. The calibration methodology applies off-axis loads to the aircraft structure to measure cross-axis structural response. The weight calculation techniques according to the invention use this calibration data to resolve cross-axis effects and compute actual aircraft weight from the structural bending strain data.